In many applications, it is necessary to deposit thin layers on a substrate. The term “substrate” as used herein shall embrace both inflexible substrates, e.g., a wafer or a glass plate, and flexible substrates, for example, webs and foils. Typical techniques for depositing layers are evaporating, sputtering, and chemical vapor deposition.
Representative examples include (but are not limited to) applications involving: semiconductor and dielectric materials and devices, silicon-based wafers, flat panel displays (such as TFTs), masks and filters, energy conversion and storage (such as photovoltaic cells, fuel cells, and batteries), solid-state lighting (such as LEDs and OLEDs), magnetic and optical storage, micro-electro-mechanical systems (MEMS) and nano-electro-mechanical systems (NEMS), micro-optic and opto-elecro-mechanical systems (NEMS), micro-optic and optoelectronic devices, transparent substrates, architectural and automotive glasses, metallization systems for metal and polymer foils and packaging, and micro- and nano-molding.
In an evaporation process, the material to be deposited is heated so that it evaporates and condenses on the substrate. Sputtering is a vacuum coating process used to deposit thin films of various materials onto the surface of a substrate. For example, sputtering can be used to deposit a metal layer, such as a thin layer of aluminum, or ceramics. During the sputtering process, the coating material is transported from a target to the substrate to be coated by bombarding the surface of the target with ions of an inert gas which have been accelerated by a high voltage. When the gas ions hit the outer surface of the target, their momentum is transferred to the atoms of the material so that some of them can gain sufficient energy to overcome their bonding energy in order to escape from the target surface and to deposit on the substrate. Thereon, they form a film of the desired material. The thickness of the deposited film is, inter alia, dependent on the duration of the substrate's exposure to the sputtering process.
Typically, sputtering systems are used to coat substrates, for example, window paints, semiconductor devices, displays, and the like. Typically, plasma is formed in a vacuum chamber, in which the sputtering target is disposed. For example, rotating sputtering targets may be used. Typically, the rotating sputtering targets have a cylindrical form and rotate about their longitudinal axis. The sputtering targets are disposed on a backing tube in which magnetrons may be arranged. The magnetrons may be driven by a direct current or an alternating current. The magnetrons are used to create the plasma in the vacuum chamber.
Typically, a magnet arrangement or rotary cathode is disposed in the backing tube. The magnet arrangement includes an inner magnet element and an outer magnet element disposed around the inner magnet element. In operation of the sputtering system, the plasma is confined in a volume, for example, above a target element if the substrate to be coated is located above the target element, between the inner magnet element and the outer magnet element, where the magnetic field is mainly parallel to the target surface. Typically, this region may be called a “race track”, as the plasma forms a closed loop with two straight parts along the long side of the magnet arrangement and a curve at both ends of the magnet arrangement. A typical arrangement of the magnet elements leads to an unbalanced situation at the ends, in particular in longitudinal direction, of the magnet arrangement, also called race track curves or plasma turn arounds. As there is more magnetic mass at the outer position, the plasma is shifted or displaced towards the inner magnet in dependence of the height above the cathode surface. This means that the plasma turn around has no stable position regarding the height above the magnet elements. Thicker targets will have a shorter race track and, therefore, a larger zone with redeposition at the end in longitudinal direction of the targets.